It is known from the German patent specification DE-3413999A, published on 7 Nov. 1985, to provide a heat exchanger in which a bundle of finned tubes includes first tubular elements comprising a series of groups of tubes, either straight or of hairpin configuration, with the groups of tubes being connected together through a second tubular element. These second tubular elements are in the form of bent tubes which define a coupling bend, so that a sealed path is defined for fluid circulating in the tube circuit thus constructed.
Such a heat exchanger may be used for the circulation of a refrigerant fluid in a heat exchanger of the phase change type, such as an evaporator or a condensor, especially in apparatus for the air conditioning of automotive vehicles.
As is mentioned in the German patent specification mentioned above, in order to ensure that the assembly of tubes and bends is sealed, a sealing joint is provided between the mating tubular elements, for example a brazed joint. For this purpose, the straight end portions of the bends are inserted into the open ends of the tubes in the corresponding groups of tubes, to which they are then secured in a sealing manner by means of a brazing operation, for example by being passed through a bath of brazing material which allows the latter to come into contact with the ends of the groups of tubes, between the outer walls of the straight end portions of the bends and the internal walls of the corresponding groups of tubes. These walls are generally expanded, in such a way as to give them a larger diameter than the outer wall of the straight end portions of the bend. This creates the space into which the braze metal can penetrate.
A problem does however remain, namely how to avoid any possible air bubbles being trapped between the telescoped portions at the open ends of the groups of tubes during the penetration of the braze metal into the interspace. The patent specification mentioned above proposes to provide two diametrically opposed vents on the straight end portions of the bend. These vents are defined by partial radial recesses or dimples which are formed in the walls of the straight end portions of the bends. They extend axially so as to bring the expanded open end of each tube in the groups of tubes into communication with the assembly between the straight end portions of the bends and the portion of the tube of each group situated beyond the assembly of the bend with the tube. In this way, the air trapped between the cooperating portions of the bend/tube assembly and the braze metal penetrating into the open end of the tube is able to escape freely through these vents.
Even so, the arrangement of the vents mentioned above still has some drawbacks. Because the vents extend into the interior of the bend, they cause some disturbance in the circulation of the refrigerant fluid circulating in the circuit, and so diminish the performance of the heat exchanger. In addition, there are two vents which prove insufficient, because air bubbles may exist in regions which are remote from the vents and which become trapped again between the walls of the tubes and bends and the braze metal.